The present invention relates to an idle speed controller for application in an internal combustion engine provided with a variable valve timing mechanism and a variable working angle mechanism, and more particularly, to an improvement of a control structure for preventing misfires when an angle advancement failure occurs in a variable valve timing mechanism.
In the prior art, a variable valve timing mechanism is used to vary the valve timing of an intake valve and an exhaust valve. The variable valve timing mechanism uses a hydraulic or electric actuator to alter the rotation angle of a camshaft relative to a crankshaft in order to vary the valve timing of an intake valve and an exhaust valve. In an internal combustion engine that uses such a variable valve timing mechanism for its intake system, misfires may occur when the engine is idling. Such misfires occur when the valve timing of the intake valve is advanced from the intended timing. Such a state is referred to as an advancement failure of the variable valve timing mechanism. The intake air amount is small when the engine is idling. If an advancement failure occurs in such a state, overlap between the periods in which the intake and exhaust valves are both open (hereafter, referred to as “valve overlap”) increases. This increases internal exhaust gas recirculation (EGR) and deteriorates combustion conditions.
To solve this problem, Japanese Patent Laid-Open Publication 2003-65131 proposes a technique for setting a target idle speed to a value higher than normal during execution of idle speed control when an advancement failure occurs in the variable valve timing mechanism. Such a technique is referred to as “idle-up control”. The engine speed is increased when the engine is idling to increase the intake air amount. This lowers the influence that an increase in internal EGR caused by an advancement failure has on combustion. This relieves the effect on the combustion conditions by the increase in the internal EGR caused by an advancement failure. Accordingly, idle-up control prevents misfires that would be caused by an advancement failure in the variable valve timing mechanism.
Japanese Patent Laid-Open Publication 2001-263015 describes a variable working angle mechanism that varies the valve working angle of intake and exhaust valves (range of crank angle from when the valve opens to when the valve closes). In an internal combustion engine that uses such variable working angle mechanism for its intake system, the amount of air drawn into a cylinder is adjusted not only by varying the throttle open amount but also by varying the valve working angle of the intake valve. Therefore, the required amount of air drawn into the cylinder is ensured through the combination of the throttle open amount and the valve working angle. For example, when the engine is operating at a predetermined throttle open amount and valve working angle, the throttle open amount may be increased or decreased. In such a case, the amount of air drawn into a cylinder is kept constant by decreasing or increasing the valve working angle of the intake valve so as to decrease or increase the drawn in air amount by the amount of air that is increased or decreased by the increase or decrease of the throttle open amount. Even though the air amount is kept constant, a fluctuation may occur in the amount of pumping loss or compression end pressure and compression end temperature (the pressure and temperature in a cylinder at the compression top dead center) depending on the combination of the throttle open amount and the valve working angle. Therefore, in an internal combustion engine using such a variable working angle mechanism for its intake system, the combination of the throttle open amount and the valve working angle is changed if necessary even when the air amount is constant. This enables the air amount to be controlled so that the internal combustion engine is operated under further favorable conditions.
In an internal combustion engine that executes air amount control in such a manner, the valve working angle of the intake valve is not always constant even when the engine is idling. Thus, depending on the setting of the valve working angle, when an advancement failure occurs in the variable valve timing mechanism, the increase amount of the internal EGR relative to an increase of valve overlap may change. In such an internal combustion engine, if the idle-up control is performed in the same manner whenever an advancement failure occurs, the idle-up amount may become excessive or insufficient. As a result, execution of unnecessary idle-up control may lower the fuel efficiency when the idle-up amount is excessive or cause misfires when the idle-up amount is insufficient.